This invention relates generally to thermocouple thermometers, and in particular to a method and apparatus for providing a calibrated isothermal assembly for a thermocouple thermometer.
It is well known in the art that a thermocouple is a pair of conductors composed of dissimilar metals and so joined at two points that a voltage is developed by the thermoelectric effects at the two junctions. Different types of metals joined together produce different thermoelectric effects, and so different types of thermocouples are commercially available, and have been for many years, for measuring different ranges of temperatures. The voltage-versus-temperature relationship of a thermocouple is nonlinear; and accurate voltage-versus-temperature tables are available for each of the various types of thermocouples. These tables were originally derived by making one of the two junctions a reference junction and placing it in an ice bath to keep the reference junction at the ice point while measuring the voltage at the other junction over a range of temperatures. Some common types of thermocouple are the type J (iron-constantan), the type K (chromel-alumel), and the type T (copper-constantan). These thermocouples are manufactured with a measuring junction at a free end for measuring temperatures, and a plug end for connecting to a measuring instrument and forming a reference junction.
Thermocouple thermometers are digital electronic instruments into which a thermocouple is plugged. The instruments typically permit selection of the thermocouple type, and include circuitry for measuring the thermocouple voltage. Conventional thermocouple thermometers also include display devices, such as liquid crystal display (LCD) devices, on which temperatures are read out in numerical characters.
The interface at which the thermocouple is plugged into the instrument is critical to the system because it is the reference junction for the thermocouple. U.S. Pat. No. 4,718,777 teaches the use of an isothermal block formed of alumina ceramic of sufficient mass and good heat conductivity to maintain stable, substantially equal temperatures at two conductive connector pads mounted inside the block where the pair of thermocouple wires plug into the instrument. A temperature sensor with a nearly linear volt-equivalent of temperature mounted inside the isothermal block between the connector pads measures the temperature of the isothermal block to compensate the output reading for error created by the reference junction. This is achieved by the instrument controller retrieving from a lookup table an error correction voltage corresponding to the measured temperature of the reference junction, and subtracting it from the voltage created by the thermocouple.
In the past, the method of calibrating the reference junction was to place the thermocouple in a lag bath at a stabilized temperature, such as room temperature, along with a mercury thermometer after the electronic thermometer instrument was manufactured. After the mercury thermometer became stabilized, the person calibrating the reference junction would, while observing the electronic thermometer's display, adjust the bias current to the temperature sensor by turning a potentiometer until the display matched the reading on the mercury thermometer. Because the temperature sensor, usually a temperature-sensing transistor, was specified with a nearly linear volt-equivalent of temperature over a temperature-measuring range and thus operated nearly linearly over a volts-versus-temperature range that was available in a lookup table, calibration was carried out at only one temperature. That is, since the voltage-equivalent of temperature of the base-emitter junction of the temperature-sensing transistor was nearly linear and specified by the manufacturer to a particular linearity tolerance, it was deemed that once calibrated at any stable temperature within the range of the thermocouple, the reference junction was calibrated for all temperatures.
The major disadvantage associated with the prior art method of calibration is that it was long and tedious, and had to be carried out after final assembly of the instrument. Each instrument had to be calibrated separately, and so the calibration times were cumulative in the manufacturing process, resulting slow, time-consuming and labor-intensive manufacture.
Another disadvantage was that because the tolerance of temperature sensors vary from part to part, even from the same manufacturer, the published tolerance specifications of the measuring instrument could be no better than the specifications published by the manufacturers of the temperature sensors.
It would be desirable to provide an isothermal thermocouple interface assembly and reference junction that could be calibrated before assembly into an instrument to obviate tedious and time-consuming calibration following manufacture of the instrument, or to permit field replacement of the interface without temperature calibration. To facilitate calibration of several isothermal assemblies at the same time, it would be desirable to make the physical size of the assembly as small as possible while still providing sufficient thermal mass and good thermal conductivity.